The 38-year-old staff sergeant Robert Bales has been depicted as a mentally strained, "rogue" killer by US and Nato military officials, who have shown Afghan officials surveillance video of his solitary return to base among other evidence that he acted alone.

Among Afghans however there is a widespread belief that the soldier had companions, and perhaps official sanction, on his shooting rampage.

The implication that the US is lying about perhaps the worst military killings in Afghanistan reflects how far a decade of spiralling war has eroded trust between the Nato-led coalition and Afghans.

"In four rooms people were killed, children and women were killed, and then they were all brought together in one room and then put on fire; that one man cannot do," Karzai told journalists, after hearing from a relative of the worst-affected family who survived because he was away that evening.

The scale of the massacre shocked the West, because although there have been far larger death tolls from air strikes, they have been accepted in foreign troops' home countries as tragic mistakes.

But for many in Afghanistan, the shooting spree is just the latest in a string of tragic and unnecessary killings by soldiers who have lots of firepower and little accountability, and who usually move in groups.

"This is not like past civilian casualty incidents. This wasn't a mistake in the heat of battle, the result of poor intelligence, or an indiscriminate reaction. But in Afghan eyes it looks pretty much the same," said Erica Gaston, a human rights lawyer working on Afghanistan and Pakistan for the Open Society Institute.

The foreign military's poor handling of other civilian deaths has made Afghans more likely to believe that there could be some kind of cover-up involved in the account of a lone, rogue gunman, she added.

"The lack of meaningful public accountability and explanation of past civilian casualties incidents has led to a widespread – if unfair – perception that international military wantonly kill Afghans with impunity."

The massacre also came after a string of damaging revelations about US soldiers' conduct in Afghanistan, that heavily undermined trust in their conduct and motives.

"I have encountered almost no Afghan who believes it could have been one person acting alone, whether they think it was a group or people back at the base somehow organising or facilitating it," said Kate Clark, of Afghanistan Analysts Network.

Rumours and conspiracy theories spread easily in Afghanistan, a country where internet is an elite luxury, and outside cities many people do not have electricity, much less television. But a conviction the US is not telling the truth about the Panjwai killings – and their wider aims in Afghanistan – is not confined to the poor or illiterate.

"The aim of the US in our country is to kill our people every day, and take our country under their control," said Mohammad Baqar Shaikhzada, a mullah and former Member of Parliament who now preaches at the Jafaryia mosque in the centre of Kabul and said he believes there were several gunmen.

"It is the job of the people of Afghanistan to show a reaction against the people of America, and that reaction has to be jihad," Shaikhzada added.

Feeding suspicion, Karzai also complained the US is not collaborating with his investigative team, as lawmakers and many other Afghans demand a speedy trial and death by hanging for the killer.

But Washington's ability to answer Afghan questions over the massacre, may be limited by its promises to bring him to justice through the complex and slow-moving US legal system.

Prosecutors in such a high-profile case are likely to be taking extreme caution to avoid any misstep that could allow defence lawyers to win Bale's freedom on a technicality, rather than a court-room examination of his guilt.

"I think the differences lie in part in the fact that it is vital that the US investigators conduct their investigation with regard for the due process rights of the suspect," said one US official who asked not to be named as he was not authorised to speak on the subject.

"The United States wants justice as much as anyone in Afghanistan for this terrible act, which has horrified every American, however the premature release of evidence could ultimately undermine prosecutorial efforts."